The present invention relates generally to an electrophotographic image-printing apparatus or machine, also commonly known as a xerographic printing apparatus, and more particularly to an improved protection circuit for monitoring the operational state of an internal scavenger plate of an electrophotographic image-printing system.
The function of the scavenger plate in a conventional electrophotographic image-printing apparatus is well known. Toner and carrier particles are mixed in a developer station or sump. The mixture (typically toner particles carried by positively charged carrier particles) is mechanically withdrawn from the sump and deposited on a toning roller, magnetically held thereon. The toning roller passes in close proximity with an image loop or photoconductor, such that the toner particles are transferred thereto in a pattern corresponding with the image to be printed (usually represented by negative charges on the image loop).
The majority of carrier particles remain on or fall back upon the toning roller. As the toning roller continues to rotate, the carrier particles, lacking toner particles, are re-deposited in the developer sump for toner replenishment.
Unfortunately, carrier particles also accumulate on the image loop. The scavenger plate interposes the toning roller and image loop, and the electrical bias on the scavenger plate draws those carrier particles off the image loop. Once released from the image loop, the carrier particles are returned to the toning roller and/or developer sump.
Failure of the scavenger plate, for whatever reason, may have severe consequences. First, carrier accumulating on the image loop may adversely affect print quality; upon release from the image loop, the carrier particles may be transferred to other rollers or the imaged paper. Second, released carrier may infiltrate other components within the electrophotographic image-printing system, causing failure. Third, accumulation rapidly depletes the supply of carrier within the developer station, interfering with the ability to control toner concentration.
Several conventional electrophotographic image-printing apparatus are shown in U.S. Pat. Nos. 4,647,186; 5,047,807; 5,184,194; and 5,715,503. The teachings thereof are fully incorporated herein by reference.
In a principal aspect, the present invention is a protection circuit for an electrophotographic image-printing apparatus having an internal scavenger plate biased by a power supply in a coupled state. The protection circuit monitors the coupled state and issues an interrupt signal whenever the coupled state is lost or absent. The protection circuit represents an improvement over conventional electrophotographic printing systems wherein an interrupt signal is provided only in response to an arc or an electrical short between the scavenger plate and the system housing.
It is thus an object of the present invention to provide a circuit for determining whether a power supply and scavenger plate of an electrophotographic image-printing apparatus are properly coupled. Another object is a protection circuit, coupled to a scavenger plate of an electrophotographic image-printing apparatus, for sensing the presence of a power cable or a bias voltage at the scavenger plate.
These and other features, objects and advantages of the present invention are described or implicit in the following detailed description of certain preferred embodiments.